This invention relates to imaging methods utilizing nuclear magnetic resonance (NMR) technique. In particular, this invention relates to a method of collecting data to reconstruct an artifact-free portion of a larger image. Additionally, in accordance with the invention, the region of interest need not necessarily coincide with the center of the scanner.
By way of background, the nuclear magnetic resonance phenomenon occurs in atomic nuclei having an odd number of protons and/or neutrons. Due to the spin of the protons and the neutrons, each such nucleus exhibits a magnetic moment, such that, when a sample composed of such nuclei is placed in a static, homogeneous magnetic field, B.sub.o, a greater number of nuclear magnetic moments align with the field to produce a net macroscopic magnetization M in the direction of the field. Under the influence of the magnetic field B.sub.o, the magnetic moments precess about the axis of the field at a frequency which is dependent on the strength of the applied magnetic field and on the characteristics of the nuclei. The angular precession frequency, .omega., also referred to as the Larmor frequency, is given by the equation .omega.=.gamma.B, in which .gamma. is the gyromagnetic ratio which is constant for each NMR isotope and wherein B is the magnetic field (including B.sub.o) acting upon the nuclear spins. It will be thus apparent that the resonant frequency is dependent on the strength of the magnetic field in which the sample is positioned.
The orientation of magnetization M, normally directed along the magnetic field B.sub.o, may be perturbed by the application of magnetic fields oscillating at the Larmor frequency. Typically, such magnetic fields designated B.sub.l are applied orthogonal to the direction of the static magnetic field by means of a radio frequency (RF) pulses through coils connected to a radio-frequency-transmitting apparatus. The effect of field B.sub.l is to rotate magnetization M about the direction of the B.sub.l field. This may be test visualized if the motion of magnetization M due to the application of RF pulses is considered in a Cartesian coordinate system (rotating frame) which rotates at a frequency substantially equal to the resonant frequency .omega. about the main magnetic field B.sub.o in the same direction in which the magnetization M precesses. In this case, the B.sub.o field is chosen to be directed in the positive direction of the Z-axis, which, in the rotating frame, is designated Z' to distinguish it from the fixed-coordinate system. Similarly, the X- and Y-axes are designated X' and Y'. Bearing this in mind, the effect of an RF pulse, then, is to rotate magnetization M, for example, from its direction along the positive Z' axis toward the transverse plane defined by the X' and Y' axes. An RF pulse having either sufficient magnitude or duration to rotate magnetization M into the transverse plane (i.e., 90.degree. from the direction of the B.sub.o field) is conveniently referred to as a 90.degree. RF pulse. Similarly, if either the magnitude or (for a rectangular pulse) the duration of an RF pulse is selected to be twice that of a 90.degree. pulse, magnetization M will change direction from the positive Z' axis to the negative Z' axis. This kind of an RF pulse is referred to as a 180.degree. RF pulse, or for obvious reasons, as an inverting pulse. It should be noted that a 90.degree. or a 180.degree. RF pulse (applied perpendicular to M) will rotate magnetization M through the corresponding number of degrees from any initial direction of magnetization M. It should be further noted that an NMR signal will only be observed if magnetization M has a net transverse component (perpendicular to B.sub.o) in the X'-Y' (transverse) plane. A 90.degree. RF pulse produces maximum net transverse magnetization in the transverse plane since all of magnetization M is in that plane, while a 180.degree. RF pulse does not produce any transverse magnetization.
RF pulses may be selective or nonselective. Selective pulses are typically modulated to have a predetermined frequency content so as to excite nuclear spins situated in preselected regions of the sample having precession frequencies as predicted by the Larmor equation. The selective pulses are applied in the presence of localizing magnetic-field gradients. Nonselective pulses generally affect all of the nuclear spins situated within the field of the RF pulse transmitter coil and are typically applied in the absence of localizing magnetic-field gradients.
There remains to be considered the use of magnetic-field gradients to encode spatial information (used to reconstruct images, for example) into NMR signals. Typically, three such gradients are necessary: EQU G.sub.x (t)=.differential.B.sub.o /.differential.x, EQU G.sub.y (t)=.differential.B.sub.o /.differential.y,
and EQU G.sub.z (t)=.differential.B.sub.o /.differential.z.
The G.sub.x, G.sub.y, and G.sub.z gradients are constant throughout the imaging slice, but their magnitudes are typically time dependent. The magnetic fields associated with the gradients are denoted, respectively, b.sub.x, b.sub.y, and b.sub.z, wherein EQU b.sub.x =G.sub.x (t)x, EQU b.sub.y =G.sub.y (t)y, EQU b.sub.z =G.sub.z (t)z,
within the volume.
In NMR imaging, it is frequently desirable to reconstruct a portion of a larger image. It may also be desirable to magnify that portion of the image which is of interest. Additionally, often the region of interest does not necessarily coincide with the center of the scanner. An example of this might be cervical or thoracic spine studies.
The transverse (e.g., in the direction of the X-axis) field of view of an NMR image is determined by the gradient amplitudes, and the rate of which the NMR signal is sampled (A/D sampling rate). For definiteness, it is beneficial to consider the case of Fourier transformation imaging schemes (such as the two-dimensional spin-warp technique to be described hereinafter, although the disclosure is generalized hereinafter to the three-dimensional case) with the X direction being the direction in which a readout magnetic-field gradient G.sub.x is applied, and the Y direction being that of a phase-encoding gradient G.sub.y. The field of view in a direction orthogonal to that of the transverse view is determined by the number of discrete phase-encoding G.sub.y gradients and their amplitudes. Increasing the amplitudes of the G.sub.x and G.sub.y gradients spreads out the object over a larger frequency and phase bandwidth. This has the result of magnifying the object in the reconstruction. However, unless the sampling rate (in the X- and Y-axis directions) is increased concomitantly, the reconstruction will exhibit aliasing, or fold-over of those portions of the object outside the field of view (i.e., those frequencies and phases which are under-sampled).
One approach which resolves the aliasing problem is to sample more rapidly in both the X direction (faster A/D rate) and in the Y direction (more phase-encoding "views"). In this case, however, for a 2.times. magnification four times the data must be acquired if the object just fills the original field of view. This is often not a satisfactory solution, since, in NMR, it is generally desired to keep data collection time as short as possible.
A means of resolving part of the under-sampling problem is to reduce the bandwidth of the NMR signal by a low-pass filter before the A/D conversion. Then, the portions of the object outside the field of view in the X direction will not alias into the reconstruction. An analogous band-limiting operation for the Y direction is not so obvious, however.
Briefly, in accordance with the invention, band limiting in the Y-axis direction is achieved by applying a selective 180.degree. RF pulse in the presence of a G.sub.y gradient. The resulting spin echo then contains phase information from only a variable-width region in the selected slice. The width of the region is dependent on the gradient and the frequency content of the selective 180.degree. pulse.
A technique using a gradient and a selective 180.degree. pulse has been previously used to collect imaging data from a line of inverted spins. The intent in line imaging is to limit data collection in the selection direction (e.g., the Y-axis direction) to a single pixel. That is, if an image has a resolution of 128 pixels in the Y-axis direction, then a signal must be obtained from 128 different lines. A linear read-out gradient in the direction of the line (i.e., the X axis) is applied to differentiate the spin frequency dependence on the position of the spin along the line. An example of such a technique is disclosed in U.S. Pat. No. 4,297,637, issued to Crooks et al.
A significant difference between the line imaging method and the inventive method for band limiting the phase of the NMR signal in the Y-axis direction is that, in accordance with the invention, pixel information in the Y-axis direction is created by Fourier transformation of the NMR signal with respect to the phase-encoding gradient amplitude or the amplitude X gradient duration product. Additionally, all of the 128 (assuming the image has 128 pixels in the Y-axis direction) NMR signals are used to obtain an improvement in the signal-to-noise (S/N) ratio of 1/.sqroot.128. In the event a 256 pixel image is reconstructed the S/N improvement would be 1/.sqroot.256. In general, if the image has N pixels in the Y-axis direction, then the signal-to-noise ratio improvement is 1/.sqroot.N. This is an advantage not realized with the line imaging methods.
It is therefore an object of the present invention to provide a method to band limit the data in the Y direction so that additional phase-encoding views are not required.
It is a further object of the invention to provide means whereby the magnified field of view can be centered at a location different than the isocenter of the gradient system.